The earliest retroreflective sheeting had an exposed lens construction, but reflex-reflection of light was inhibited when the lenticular surfaces of the lenses were covered with water. This problem was answered by enclosed-lens retroreflective sheeting in which, as first taught in U.S. Pat. No. 2,407,680 (Palmquist et al.), the lenses were totally embedded within a sheeting that had a flat front surface provided by a flat transparent top film. This allowed incident light rays to be focused onto the specularly reflective layer irrespective of whether the front of the sheet was wet or dry.
U.S. Pat. No. 3,190,178 (McKenzie) solved the same problem in a different way, namely, by modifying retroreflective sheeting of the exposed-lens type wherein the lenses are partially embedded in a binder layer. In the McKenzie patent, the exposed lenses are protected by a cover film or sheet to which the binder layer is sealed along a network of interconnecting lines, thus forming a plurality of hermetically sealed cells within which the lenses are encapsulated and have an air interface. Such exposed-lens sheeting is sometimes referred to as "encapsulated-lens" or "cellular" retroreflective sheeting. U.S. Pat. No. 3,190,178 discloses a cellular retroreflective sheeting that is formed from (1) a base sheet comprising retroreflective elements partially embedded in a binder layer and (2) a cover sheet. The sheeting is formed by lamination of the base sheet and cover sheet together with heat and pressure to displace binder material from the binder layer into adherent contact with the cover sheet. The displacement is performed in a pattern to provide the desired network of interconnecting bonds that define the hermetically sealed cells. An advance upon this technique is disclosed in U.S. Pat. No. 4,025,159 (McGrath) wherein the binder material is taught to be a thermoformable material that is cured in situ after being thermoformed, thereby achieving more reliable adhesion of the binder material to the cover sheet.
The front portion of such retroreflective sheeting, on which light to be reflected is received, is made up of individual cells of retroreflecting elements, e.g., microspheres having an aluminum vapor coat on the rear surfaces thereof, separated by the pattern of interconnecting bonds that define them. The individual cells may be of any shape, but are typically of uniform, regular shape, e.g., squares or hexagons, to enable easier fabrication and to improve the appearance of the sheeting. Typical cells are on the order of 3 to 4 millimeters wide. The interconnecting bonds that define and separate the cells, sometimes referred to herein as "seal legs", are typically on the order of 0.5 millimeters wide. According to the aforementioned references, as the base sheet and cover sheet are laminated together the binder material swallows up or flows around the microspheres in the seal legs. While wider bonds may be desired to provide greater resistance to delamination, such measures will reduce the proportion of the total surface of the sheet which is retroreflective, thereby reducing the total brightness of the sheeting. Typically, the seal legs make up about 20 to 30 percent of the total surface area of the retroreflective sheeting with the remaining portion being made up by the cells of retroreflective elements.
It is often desired that retroreflective sheeting have a white appearance under ambient conditions. A white appearance is typically preferred for aesthetic reasons as well as for functional reasons, e.g., to provide an effective contrast between the indicia and background of a sign such as a speed limit sign. As indicated above, about 70 to 80 percent of the surface area of a typical retroreflective sheeting is made up of the cells containing several hundreds or thousands of vapor-coated microspheres. The microspheres are typically gray in appearance due to the aluminum vapor coat on the rear surfaces thereof. Whiteness of a sheeting may be improved be incorporating a whitening agent, e.g., a pigment such as titanium dioxide, in the binder layer or cushion coat of the base sheet such that the material that flows around the microspheres in the seal legs into contact with the cover sheet will have a white appearance. To a lesser extent, increasing the whiteness of the binder material in this fashion may also tend to increase the whiteness in appearance of the cells of the retroreflective sheeting wherein minute portions of the binder layer may be visible between individual microspheres. The major influence upon overall sheeting whiteness is, however, typically provided by the seal leg areas. The overall whiteness of retroreflective sheeting is commonly measured or expressed in terms of Cap Y which may be determined according to ASTM E97-77. For instance, U.S. Department of Transportation Federal Highway Administration Specification FP85 Section 718.01 Retroreflective Sheeting Materials provides that Type II retroreflective sheetings (i.e., encapsulated-lens retroreflective sheetings) have a CAP Y of at least 27.
In order to enhance the flexibility of retroreflective sheeting, it may be advantageous to use a cover sheet that comprises relatively more flexible materials than the first disclosed cover films which comprised such materials as polymethylmethacrylate. Such materials may also offer other advantages when used as cover films, e.g., improved impact resistance or greater impermeability to moisture. An example of such a cover film is one comprising ethylene/acrylic acid copolymer such as is disclosed in U.S. Pat. No. 4,896,943 (Tolliver et al.), which is commonly assigned herewith.
It has been found, however, that cover film materials that are relatively more flexible and exhibit thermoplastic characteristics, particularly during fabrication of retroreflective sheeting, may tend to soften at relatively lower temperatures than do relatively less flexible cover films. Thus, flexible cover films that are extrudable, i.e., are thermoplastic, rather than being solvent-cast, may be subject to softening to a significant degree during the lamination and sealing of the cover film to a base sheet comprising the retroreflective elements. For example, polyolefin-based cover films may be preferred because of the advantages, e.g., conformability, impact resistance, moisture impermeability, high flexibility, clarity, and strength, which they can impart to a cover film and retroreflective sheeting incorporating same. During a typical lamination process, the binder layer is heated to about 220.degree. to 350.degree. F. (105.degree. to 195.degree. C.) by a patterned pressure roll that forces the binder material, according to the pattern, into adherent contact with the inside surface of the cover film, which is in turn believed to be heated by the warmed binder layer to about 150.degree. to 160.degree. F. (65.degree. to 70.degree. C.), i.e., temperatures sufficiently warm to change the surface characteristics, e.g., degree of softness and/or tendency to adhere to such materials as glass microspheres, of some cover films. It has also been found, that when cellular retroreflective sheeting is formed by such lamination techniques as disclosed in the aforementioned McKenzie and McGrath patents, e.g., with heat and pressure, if the cover film comprises a material such as a polyolefin-based copolymer, e.g., ethylene/acrylic acid copolymer that tends to soften and/or adhere to the surface of the microspheres at the temperatures at which the thermoforming is performed, that the resultant sheeting may not be as white as might be desired, i.e., the Cap Y of the resultant sheeting is not high enough. In such sheetings it has been found that the seal legs tend to have a substantially gray appearance that is believed to be the result of the microspheres in the seal leg not being effectively swallowed up or flooded by the white binder material when the sheeting is laminated to force the binder material into adherent contact with the cover sheet. Thus the gray aluminum vapor coat on the rear surface of the microspheres retains visible and detracts from the overall whiteness of the resultant retroreflective sheeting.